The present invention relates to a vertebral implant. More particularly this invention concerns such an implant and a tool for setting it in place and adjusting it.
When a vertebra is broken or crushed it is frequently necessary to ablate it. Then, in order to prevent the spinal column from collapsing with damage to the fragile spinal cord running in the vertebral foramen forward of the vertebral body, it is necessary to employ a spacer. This device is braced vertically between the bodies of the adjacent vertebra and holds them apart at the desired spacing.
U.S. Pat. No. 5,571,192 describes such an implant which comprises a tubular center element extending along an axis and a pair of end elements. The center element is formed with upper and lower screwthreads of opposite hand and with a plurality of radially throughgoing apertures. The upper and lower tubular end elements are each formed with a plurality of radially through-going aperture, each have a circular-section inner end threaded onto a respective one of the screwthreads, and each also have an outer end adapted to bear on a respective one of the adjacent vertebrae.
Thus such an implant can be set in an area where the body or bodies or one or more vertebra have been ablated. The length of the implant is then increased by rotating the center element to force out the end elements and bring their outer ends into solid engagement with the confronting vertebral surfaces. The screwthreads offer sufficient mechanical advantage that the system can even be used to distract the vertebrae, as is frequently necessary in the event of a crushing injury to a vertebra. The tubular elements of the implant can be filled with bone cement and/or bone fragments to ensure that the implant becomes anchored in place in living bone. Since the outer elements surround the screwthreads of the inner element, once installed the screwthreads will be largely covered so that their sharp edges do not impair healing.
Such an implant has proven very effective in use. It is, however, fairly difficult to position and expand. Thus the surgical field must normally be fairly wide in order to permit the surgeon to get at the center element with a tool and rotate it. Even if each angular stepping of this center element is only through a relatively small angle, it is still necessary to open up the patient quite a bit. Obviously this is not always possible or advisable.
It is therefore an object of the present invention to provide an improved vertebral implant.
Another object is the provision of such an improved vertebral implant which overcomes the above-given disadvantages, that is which can be set in place and expanded in a relatively reduced surgical field.
A further object is to provide a tool usable with this implant which expands it axially and which requires that the opening be only big enough to pass the implant.
A spinal implant for engagement between a pair of vertebrae has according to the invention a pair of parts displaceable relative to each other along an axis and each adapted to engage a respective one of the vertebrae. One of the parts is formed with a screwthread engaging a nut that bears axially on the other of the parts and that is formed with an externally accessible array of gear teeth.
It is possible to rotate this nut by means of a tool that can engage through a relatively narrow opening, so that the operating field need merely be wide enough to allow the implant to pass through.
The parts according to the invention are tubular and coaxial. The one part having the screwthread is inside the other part so their interior can be filled with bone chips and/or cement to knit the implant with the vertebrae it is engaging. In addition interengaging formations on the parts preventing relative rotation about the axis. These formations include a radially open and axially extending groove and a radially projecting pin engaged in the groove.
The other outer part has a rim against which the nut fits and is formed with a radially throughgoing notch opening at the rim and having a frustoconical surface acting as a seat for a bevel gear of the setting tool. It is also formed diametrically across from the notch with a radially open threaded hole. The one inner part is formed in line with the notch and hole of the other outer part with respective axially extending and radially throughgoing slots. When bone grows through these slots, it effectively blocks the two parts from rotating relative to each other and thereby shortening the implant.
The other outer part in accordance with the invention is formed with a pair of outwardly open holder grooves symmetrically flanking a plane extending along the axis and bisecting the notch and threaded hole. Furthermore the flange is formed with an array of throughgoing holes. Such a flange allows bone cement to be applied to the end of the implant without getting on the setting tool, and the holes ensure that the flange will bond solidly to the vertebra it engages. The one inner part is formed with a radially projecting flange and the nut lies between the flange and the rim. Furthermore the nut is formed with an axially extending spacer collar engageable axially with the one inner part. The flange is spaced from the one inner part when the collar engages the one inner part.
Such an implant is used according to the invention with a setting tool having a holder fittable with the other part, a rotary member in the holder, and a gear on the rotary member meshable with the teeth of the one part when the holder is fitted to the other part. This rotary member is a rod having one end provided with the gear and an opposite end provided with a hand wheel. The rod is tubular and coaxially receives a core rod having an inner end screwable into the other part. The tool further has an outer tube coaxially surrounding the shaft and having an outer end carrying the holder. The holder is a fork engageable around the other part. This fork is double and has four tines engageable with the other part.
Such a tool can be solidly locked to the implant, with the double fork gripping the outer part and preventing it from rotating about the axis of the tool and the core rod threaded into the outer part also locking it to the tool, so that the implant can be manipulated without any possibility of the coming off the tool. With the core rod extending diametrally through the inner implant part, any relative rotation of these two parts is impossible. Considerable torque can be applied to the core rod to rotate the bevel gear and extend the implant also without any fear of damage to surrounding tissues, since the surgeon can hold the outer tubular shaft in one hand to prevent any rotation of it while turning the core rod with the other hand to expand the implant. This prevents any damage to tissues during the delicate expansion or distraction step.